464 SCIENTIFIC RECORD FOR 1884. 



extraordinary index for these three lines was 2.8415, 2.9029, and 2.9817, 

 respectively. The only minerals having higher indices are zigueline, 

 red silver, and cinnabar. {Zeitschr. Kryst. & 3Iin., vii, '167 ; J. Phys., 

 February, 1884, II, iii, 105.) 



Soret has determined the refractive indices of the alums for the eight 

 principal lines of the spectrum. He used seven alumina alums, four 

 chrome alums, five iron alums, two gallium alums, and an indium alum. 

 {G. R., November, 1884, xcix, 8(57.) 



Quincke has investigated the change which takes place in the volumes 

 and refractive indices of liquids when subjected to h^^drostatic press- 

 ure. The liquids were contained in glass or metal tubes 230"""' long, 

 which were placed in an interference apparatus. One of the two inter- 

 fering pencils traversed the liquid, and by measuring the number of in- 

 terference bands which corresponded to a given Fraunhofer line, before 

 and after the j^ressure was applied, the change in the index could be 

 calculated. He draws the conclusion that at constant temperature and 

 varying hydrostatic pressure, the specific refraction, at least for the 

 liquids examined, which is .w — 1 — ff, is equal to a constant; in other 

 words, the decimals of the refractive index are proportional to the den- 

 sity. {Phil. Mag., January, 1884, V, xvii, 65.) 



Shaw has described a means of verifying the phenomena of refraction 

 by the prism, especially the focal lines, by using a piece of wire gauze 

 as the object, placed so that one set of wires is horizontal and the other 

 vertical, and illuminated by a sodium flame behind it. If the light pass 

 directly from the gauze to the prism, the focal lines are of course vir- 

 tual, but they may be easily viewed and their positions identified by 

 means of a telescope which will focus an object at a short distance. For 

 one position of the eye-piece of the telescope the vertical wires are seen 

 distinctly, but not the horizontal wires; while for another position these 

 latter may be seen, but the former ones are not visible unless the prism 

 is at minimum deviation. The experiment is more striking if the fo'tal 

 lines be made real by interposing between the gauze and the prism 

 a convex lens of somewhat long focus. The vertical and horizontal 

 images may then be viewed by means of an ordinary magnifier, or, bet- 

 ter, by a telescope eye piece placed behind a second gauze, with its wires 

 at 450 to the vertical. In this way the images corresponding to the two 

 focal lines can be seen very clearly, and their distances from the prism 

 accurately measured. If the prism be placed first in the position of 

 minimum deviation, and the magnifier be focused upon the image of the 

 gauze, both horizontal and vertical wires are seen sharply defined. On 

 gradually turning the prism the vertical lines disappear completely. 

 If the eye-piece be drawn back some way, a badly defined image of the 

 gauze can be obtained, corresponding to the circles of least confusion; 

 and on withdrawing the eye-piece still farther, the horizontal wires dis- 

 appear entirely, while the vertical ones come out sharply as a set of bars 

 across a uniform field. (Nature, December, 1884, xxxi, 185.) 



